1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more specifically, to a surface acoustic wave device which generates BGS waves therein for use in high-frequency resonators and high-frequency filters.
2. Description of the Related Art
A surface acoustic wave device is generally made of a substrate having piezoelectricity ("a piezoelectric substrate") and a pair of comb-shaped electrodes having a plurality of electrode fingers arranged on the substrate so that the electrode fingers are interdigitated with one another. Such a surface acoustic wave device generates a surface acoustic wave when an electric signal is applied to the interdigital transducer.
As a surface acoustic wave, Rayleigh waves are well known, but BGS waves (Bleustein-Gulyaev-Shimizu waves or piezoelectric surface shear waves) and Love waves have also been used.
In these surface acoustic waves, a resonant frequency and electrical and mechanical characteristics thereof, such as an electrical or mechanical quality factor "Q" and an electromechanical coupling factor "k," are roughly determined by the material quality of the piezoelectric substrate and the configuration of the interdigital transducer. Uses of such surface acoustic waves for high-frequency resonators and high-frequency filters are possible.
A surface acoustic wave literally is an elastic surface acoustic wave that propagates concentrated energy in the vicinity of the surface of a substrate. There are only two components of a displacement of the surface acoustic wave propagating along a surface on an isotropic substrate; one component in the wave progress direction and one component in the direction of the substrate thickness. There is no surface acoustic wave (an SH wave or a horizontally-polarized shear wave), corresponding to a transverse wave, having only one component in a direction that is perpendicular to the wave propagation direction and parallel to the substrate surface in the isotropic substrate.
However, since the piezoelectric substrate is anisotropic, it can propagate the SH wave (including a pseudo SH wave) with concentrated energy in the vicinity of the surface thereof. This surface acoustic wave is called a BGS wave. Since the BGS wave is completely reflected by an edge of the piezoelectric substrate in the wave propagation direction, it is not required that a reflecting device (a reflector) be disposed on the substrate as in a surface acoustic wave device propagating Rayleigh waves. Therefore, the BGS wave has an advantage over the Rayleigh wave with respect to miniaturizing a surface acoustic wave device.
However, the BGS wave using a substrate having a small electromechanical coupling factor "k" cannot achieve sufficient filter characteristics when a narrow band filter is formed because an impedance ratio Za/Zr, which is a ratio between an impedance Za at an anti-resonant frequency and an impedance Zr at a resonant frequency, is comparatively small. On the other hand, the BGS wave using a substrate having a large electromechanical coupling factor "k" has very poor resistance to external conditions, although the impedance ratio Za/Zr is large so that a wide band filter can be provided.
In addition, it is desirable to be able to select various piezoelectric materials used to form the piezoelectric substrate so as to increase design choices associated with forming a surface acoustic wave device.